1. Technical Field
The present disclosure relates to a reflection mask for extreme ultraviolet (EUV) photolithography and a method of fabricating the same, and more particularly, to a reflection mask for EUV photolithography and a method of fabricating the reflection mask, wherein a high resolution pattern can be formed via destructive interference of unnecessary EUV light by forming an additional multi-layer, which can reverse a phase of the EUV light, and wherein the additional multi-layer is formed between a reflection layer and an absorption layer.
2. Discussion of the Related Art
Recently, the large integration scale of semiconductor devices has led to an increased use of light of shorter wavelengths in a photolithography device fabrication process. In a conventional photolithography method, EUV light is used to realize a design size of 100 nm or less.
Since most substances have a high light absorptivity at a EUV ray region, photolithography using EUV light requires a reflection mask that is different than a typical mask. A conventional reflection mask for EUV light is obtained by forming a pattern, made of a material that can absorb EUV light, on a reflection surface having a high reflection index to EUV light. The region of the reflection surface where the pattern of the absorption material is formed constitutes an EUV light absorption region, while an exposed region of the reflection surface where the pattern of the absorption material is not formed constitutes an EUV light reflection region. EUV light reflected from the reflection surface is transmitted to a photoresist material on a substrate in order to form a photoresist pattern in the same shape as the pattern of the absorption material. This is typically how the photolithography process for semiconductor fabrication is performed.
FIG. 1 is a cross-sectional view illustrating a structure of a reflection mask 1 for EUV photolithography.
The reflection mask 1 includes a substrate 2, a reflection layer 4 formed on the substrate 2, and an absorption material pattern 6 formed on the reflection layer 4. The substrate 2 is formed of a material such as silicon or glass.
The reflection layer 4 is formed in a multi-reflection layer structure, wherein different types of layers such as molybdenum and silicon (Mo/Si) or Beryllium and silicon (Be/Si) are arranged alternately over each other. The absorption material pattern 6 is formed of tantalum nitride that can absorb EUV light, and a layer of tantalum nitride is applied in a pattern to make an absorption region for EUV light.
The absorption material pattern 6 has a height H1 of approximately 100 nm. Since the height H1 of the absorption material pattern 6 is relatively large, an impinging angle θ1 also becomes large. Thus, quite often, a photoresist layer applied to a thin film 10 on a silicon wafer 8 that is being manufactured cannot be exposed to the EUV light reflected from the reflection layer 4 and, thus, an intended pattern shape cannot be obtained. This event is called “a shadow effect.”
Performing an EUV photolithography process using the reflection mask 1 may cause a size D1 of the absorption material pattern 6 and a size D2 of a photoresist pattern 12 formed on the silicon wafer 8 to be different from each other. In this exemplary case, the size D1 of the absorption material pattern 6 is larger than the size D2 of the photoresist pattern 12.
Since the photoresist layer applied on the thin film 10 is overly exposed to EUV light, the height of the photoresist pattern 12 is lowered. Thus, in many cases, the photoresist pattern 12 may not function as a mask in a subsequent etching process. The cause of this limitation is that the absorption material pattern 6 does not absorb EUV light completely and, thus, the photoresist layer formed on thin film 10 applied to the silicon wafer 8 is exposed to EUV light a reflected from the surface of the absorption material pattern 6 and to EUV light b transmitted through the absorption material pattern 6 and then reflected from the surface of the reflection layer 4.
In other words, due to the shadow effect and the unnecessary EUV light beams a and b, the photoresist on the thin film 10 layer results in a photoresist pattern 12 that is formed to have different width or height from the intended ones. Hence, when an etching process is performed using the photoresist pattern 12 as a mask, a pattern is formed on the thin film layer 10 having a different width or height from the intended pattern.